Global warming predicted by United Nations Intergovernmental Panel on Climate Change (IPCC) climate models are “inconsistent” with observed temperature trends, according to a study.
A new study by climate scientists Nic Lewis and Judith Curry found “multicentennial or multidecadal future warming under increasing forcing of only 55−70% of the mean warming simulated by CMIP5 models” used by the IPCC,” Lewis wrote.
It’s the latest in a slew of studies trying to figure out how much warming can be expected from a doubling of carbon dioxide levels. The answer: not much, according to Lewis and Curry.
The study was an update to research Lewis and Curry conducted in 2014 to estimate the climate’s sensitivity to a doubling of carbon dioxide levels in the atmosphere — a measurement known as equilibrium climate sensitivity (ECS).
Lewis and Curry’s 2014 study pegged ECS at 1.64 degrees Celsius. The IPCC’s latest climate assessment, released in 2013, put sensitivity between 1.5 and 4.5 degrees Celsius. But IPCC models typically have an ECS around 3 degrees Celsius.
Incorporating new evidence, Lewis and Curry came up with an ECS of just 1.5 degrees Celsius, which is about 10 percent below what they found a few years ago. If their results hold, it means the IPCC’s models overestimate future warming by 30 to 45 percent.
“These results imply that high ECS and [transient climate response] values derived from a majority of CMIP5 climate models are inconsistent with observed warming during the historical period,” Lewis wrote in a blog post summarizing the study’s results.
Lots of papers have been put out to try to narrow the IPCC’s range of potential ECSs — the IPCC’s 2013 report declined to give an ECS “because of a lack of agreement on values across assessed lines of evidence.”
A study published in January 2018 claimed to have narrowed the range of how much warming to expect from a doubling of CO2 concentrations, ruling out the highest and lowest estimates. Effectively, the study found a “worst-case” global warming scenario was less likely.
“Our study all but rules-out very low or very high climate sensitivities, so we now know much better what we need to,” lead author Peter Cox from the University of Exeter said in a statement.
Cox’s study shrinks the range of climate sensitivity from 1.5 to 4.5 degrees Celsius presented by the IPCC to a range of 2.2 to 3.4 degrees. Cox’s central ECS estimate is 2.8 degrees.
However, Lewis and Curry present an even lower ECS than Cox. Lewis and Curry based their ECS estimate on the baseline years between 1869 and 1882 and end dates from 2007 to 2016, “which provides the largest change in forcing and hence the narrowest uncertainty ranges,” Lewis argued.
Lewis noted the ECS “reductions stem primarily from a significant upwards revision in estimated methane forcing following more accurate determination of the forcing-concentration relationships for the principal well-mixed greenhouse gases and revisions to post-1990 [IPCC 2013] aerosol and ozone forcing estimates that reflect updated emission data, partially offset by a 2.5% upwards revision in the forcing from a doubling of pre-industrial carbon dioxide.”
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